exec(S) 6 January 1993 exec(S) Name exec: execl, execv, execle, execve, execlp, execvp - execute a file Syntax cc . . . -lc int execl (path, arg0, arg1, ..., argn, (char *)0) char *path, *arg0, *arg1, ..., *argn; int execle (path, arg0, arg1, ..., argn, (char *)0, envp) char *path, *arg0, *arg1, ..., *argn, *envp[ ]; int execlp (file, arg0, arg1, ..., argn, (char *)0) char *file, *arg0, *arg1, ..., *argn; int execv (path, argv) char *path, *argv[]; int execve (path, argv, envp) char *path, *argv[], *envp[]; int execvp (file, argv) char *file, *argv[]; Description The exec system call in all its forms transforms the calling process into a new process. The new process is constructed from an ordinary, execut- able file. Executable files consist of a header (see a.out(FP)), a text segment, and a data segment. The data segment contains an initialized portion and an uninitialized portion (bss). There can be no return from a successful exec because the calling process is overlaid by the new pro- cess. Executables created by C programs always use the function main as their entry point. The prototype for main is (arguments are optional): main(argc, argv, envp) int argc; char **argv, **envp; where argc is the argument count, argv is an array of character pointers to the arguments themselves, and envp is an array of character pointers to the environment strings. As indicated, argc is conventionally at least one, and the first member of the array points to a string contain- ing the name of the file. The path argument points to a path name that identifies the new process file. The file argument points to the new process file. The path prefix for this file is obtained by a search of the directories passed as the environment line "PATH="; see environ(M). The environment is supplied by the shell; see sh(C). arg0, arg1, ..., argn are pointers to null-terminated character strings. These strings constitute the argument list available to the new process. By convention, at least arg0 must be present and point to a string that is the same as path (or its last component). argv is an array of character pointers to null-terminated strings. These strings constitute the argument list available to the new process. By convention, argv must have at least one member, and it must point to a string that is the same as path (or its last component). argv is ter- minated by a null pointer. envp is an array of character pointers to null-terminated strings. These strings constitute the environment for the new process. envp is ter- minated by a null pointer. For execl and execv, the C run-time start-off routine places a pointer to the environment of the calling process in the global cell: extern char **environ; and it is used to pass the environment of the calling process to the new process. File descriptors open in the calling process remain open in the new pro- cess, except for those whose close-on-exec flag is set; see fcntl(S). For those file descriptors that remain open, the file pointer is unchanged. Signals set to terminate the calling process are set to terminate the new process. Signals set to be ignored by the calling process are set to be ignored by the new process. Signals set to be caught by the calling pro- cess are set to terminate the new process; see signal(S). For signals set by sigset, exec ensures that the new process has the same system signal action for each signal type whose action is SIGDFL, SIGIGN, or SIGHOLD as the calling process. However, if the action is to catch the signal, then the action is reset to SIGDFL, and any pending signal for this type is held. If the set-user-ID mode bit of the new process file is set (see chmod(S)), exec sets the effective user ID of the new process to the owner ID of the new process file. Similarly, if the set-group-ID mode bit of the new process file is set, the effective group ID of the new process is set to the group ID of the new process file. The real user ID and real group ID of the new process remain the same as those of the cal- ling process. The shared memory segments attached to the calling process are not attached to the new process; see shmop(S). Profiling is disabled for the new process; see profil(S). The new process also inherits the following attributes from the calling process: + nice value; see nice(S) + process ID + parent process ID + process group ID + semadj values; see semop(S) + tty group ID; see exit(S) and signal(S) + trace flag; see ptrace(S) request 0 + time left until an alarm clock signal; see alarm(S) + current working directory + root directory + file mode creation mask; see umask(S) + file size limit; see ulimit(S) + utime, stime, cutime, and cstime; see times(S) + file-locks; see fcntl(S) and lockf(S) The exec system call fails and returns to the calling process if one or more of the following is true: [E2BIG] The number of bytes in the new process's argument list is greater than the system-imposed limit of 5120 bytes. [EACCES] + Search permission is denied for a directory listed in the new process file's path prefix. + The new process file is not an ordinary file. + The new process file mode denies execution permission. [EAGAIN] Not enough memory. [EFAULT] + Required hardware is not present. + path, argv, or envp point to an illegal address. [EINTR] A signal was caught during the exec system call. [ELIBACC] Required shared library does not have execute permission. [ELIBEXEC] Trying to exec a shared library directly. [EMULTIHOP] Components of path require hopping to multiple remote ma- chines. [ENOENT] One or more components of the new process path name of the file do not exist. [ENOEXEC] The exec is not an execlp or execvp, and the new process file has the appropriate access permission but an invalid magic number in its header. [ENOLINK] path points to a remote machine and the link to that ma- chine is no longer active. [ENOMEM] The new process requires more memory than is allowed by the system-imposed maximum MAXMEM. [ENOTDIR] A component of the new process path of the file prefix is not a directory. [ETXTBSY] The new process file is a pure procedure (shared text) file that is currently open for writing by some process. Notes Security services provided by the operating system provide the following: + Executing a setuid/setgid file without a valid login UID fails. + The login UID is inherited from the parent process. See also a.out(FP), alarm(S), environ(M), exit(S), fcntl(S), fork(S), lockf(S), nice(S), ptrace(S), semop(S), sh(C), signal(S), sigset(S), times(S), ulimit(S), umask(S) Diagnostics If exec returns to the calling process, an error has occurred; the return value is -1 and errno is set to indicate the error. Standards conformance execl, execle, execlp, execv, execve and execvp are conformant with: AT&T SVID Issue 2; X/Open Portability Guide, Issue 3, 1989; IEEE POSIX Std 1003.1-1990 System Application Program Interface (API) [C Language] (ISO/IEC 9945-1); and NIST FIPS 151-1.